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  • Author or Editor: C.B. Rajashekar x
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Heat tolerance and endogenous ABA levels in leaves and cultured grape cells (Vitis spp., cultivars Venus and Veeblanc) were evaluated during beat acclimation. Plants and cultured cells were acclimated at 38 and 36C, respectively. Heat tolerance increased rapidly after exposing plants or cells to acclimation temperatures, reaching a maximum after 10 to 16 hours and 10 to 12 hours for leaves and cultured cells, respectively. Free and bound ABA levels increased sharply during the first hour of heat acclimation, before leaves and cultured cells reached their maximum beat tolerance. The increase in ABA during heat acclimation was 2- to 3-fold that of the nonacclimated control, and the time of the ABA accumulation peak in tissue roughly corresponded to the maximum heat tolerance in leaves and cultured cells. Heat tolerance was induced in cultured cells by exogenous ABA application. Heat tolerance increased significantly after 24 hours of ABA application at 7.6 or 9.5 μm. The results suggest that ABA may be a factor in high-temperature acclimation and beat-tolerance induction in grapes. Chemical name used: abscisic acid (ABA).

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Seasonal changes in deep supercooling and cold-hardiness of stem tissue and apical buds of pecan [Carya illinoensis (Wang enh.) C. Koch] cultivars were studied. All the pecan cultivars showed supercooling in stem and apical buds. Supercooling in stem and apical buds was maximal in early January and least in early spring. A good correlation between killing temperatures and freezing of supercooled water was found in apical buds. Similar results were observed for stem samples collected during early spring. Apical buds appeared to be more prone to injury during spring than stem tissue in all the pecan cultivars. In early April, stem samples of pecan cultivars were killed at or below –20.1C, whereas apical buds were killed at –16C or above. Apical buds of ‘Posey’ showed greater cold-hardiness than those of other pecan cultivars in midwinter and early spring.

Open Access

Effects of hypoxia on germinating bean seeds (Phaseolus vulgaris cv. Tendergreen) were examined by imbibing them in water for various lengths of time. Hypocotyl elongation under hypoxic conditions and recovery from hypoxia in bean seeds were determined. Oxygen concentration in the water began to decrease sharply after 12 h of seed imbibition and had declined by more than 63% after 3 days of seed imbibition. When seeds were germinated on 0.8% agar after 24 h of imbibition, the hypocotyl elongation was reduced by about 70% compared to the seeds with no hypoxia, and longer imbibition resulted in poor or no germination. Exogenous hydrogen peroxide (20 mm) in water increased the oxygen concentration from 250 to 350 mm in the presence of seeds and was considerably higher after 3 days of seed imbibition than that in the control. Hypocotyl elongation occurred in seeds submerged in water containing hydrogen peroxide up to 72 h while none was observed in water. This was comparable to hypocotyl elongation under non-hypoxic conditions. Hypoxia in imbibing seeds was overcome by the high oxygen levels in the medium resulting from reaction of hydrogen peroxide with seed catalase and catalytic metal ions. Considerable catalase activity was detected in germinating seeds and the use of a catalase inhibitor, aminotriazole, suggests that the enzyme plays an important role in the release of oxygen into the medium. Of the catalytic metals, the seed content of iron was dominant and was about 6 folds higher than that of either copper or manganese.

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Demand for organically grown produce is increasing, largely due to concerns of consumers about health and nutrition. Previous studies have not shown a consistent difference of essential nutrients, such as vitamins and minerals, between organic food crops and the conventional counterparts. However, to date, little consideration has been given to phytochemicals, secondary plant metabolites with potential health-promoting properties. We first discuss factors that can infl uence the levels of phytochemicals in crops, and then we critically review the results of published studies that have compared the effects of organic and conventional production systems on phytochemical contents of fruit and vegetables. The evidence overall seems in favor of enhancement of phytochemical content in organically grown produce, but there has been little systematic study of the factors that may contribute to increased phytochemical content in organic crops. It remains to be seen whether consistent differences will be found, and the extent to which biotic and abiotic stresses, and other factors such as soil biology, contribute to those differences. Problems associated with most studies tend to weaken the validity of comparisons. Given the limitations of most published studies, needs for future research are discussed.

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In a growth chamber study, lettuce (Lactuca sativa) plants were used to evaluate the effects of water deficits on health-promoting phytochemicals with antioxidant properties. Lettuce plants were treated with water stress by withholding water once at 6 weeks after sowing for 2 days or multiple times at 4 weeks for 4 days, at 5 weeks for 3 days, and at 6 weeks for 2 days. Water stress increased the total phenolic concentration and antioxidant capacity in lettuce. Young seedlings, 7 days after germination, had the highest total phenolic concentration and antioxidant capacity, and also, younger plants were typically more responsive to water stress treatments in accumulating the antioxidants than older plants. Phenylalanine ammonia lyase and γ-tocopherol methyltransferase genes, involved in the biosynthesis of phenolic compounds and vitamin E, respectively, were activated in response to water stress, although no activation of L-galactose dehydrogenase was detected. Lettuce plants subjected to multiple water stress treatments accumulated significant amounts of chicoric acid compared with the control plants. Although the increase in antioxidant activity in water stress-treated plants at harvest was not as great as in young seedlings, it was significantly higher than the control. One-time water stress treatment of lettuce at the time of harvest did not result in any adverse effect on plant growth. Thus, these results show that mild water stress in lettuce applied just before harvest can enhance its crop quality with regard to its phytochemical concentration without any significant adverse effect on its growth or yield.

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